Civil Engineering

The CMS cavern, at Point 5 of the LHC near Cessy, was excavated from scratch in an old LEP (a previous CERN accelerator, the Large Electron Positron Collider) access point. The work, which took six and a half years, finishing in February 2005, consisted of creating two caverns 100m underground, including the 53-metre long, 27-metre wide and 24-metre high experiment cavern, as well as two new shafts.

CMS is like a cylindrical onion built around the beam pipe, but cut up into 15 slices. The experiment employed a unique method of construction: assembling and testing each of the slices of detector on the surface before lowering them underground, rather than building the components within the cavern itself. This meant having to transport and lower huge but delicate pieces of detector, weighing as much as 2000 tonnes, down a 100-metre shaft but also gave a number of advantages. Not only did it enable civil engineering work on the cavern to go on in parallel with detector construction, but also the ‘slicing’ means that each piece remains accessible for ease of maintenance within the cavern.

Because of this method, CMS was in a unique position to be able to excavate where geology was difficult, without any delays hampering the progress of the detector itself. And at Cessy the ground is indeed difficult: 75 metres of “moraine”, a glacial mixture of sand and gravel containing two water tables, followed by “molasse” rock, a form of soft sandstone. To excavate through this loose earth and soft rock and to build a cavern without it collapsing was an enormous challenge. To find out how they did it, click here